1. Field of the Invention
The present invention relates to a color filter used, for example, in a color liquid crystal display device and a liquid crystal display device provided with the same.
2. Description of the Related Art
In recent years, a color liquid crystal display device has been used in various applications such as a monitor for personal computers (PCs), a display for cell phones, laptop PCs and personal digital assistants, because of advantages such as lower power consumption and space saving, and has also been used in liquid crystal televisions in recent years as substitutes for conventional CRT-based televisions.
Color reproducibility is considered important in liquid crystal televisions.
The color reproducibility of a color liquid crystal display device is determined by the colors of lights emitted from red, green and blue filter segments constituting a color filter and is evaluated on the basis of the area of a triangle (color reproducing area, hereinafter “area A”) defined by three chromaticity points of red, green and blue filter segments expressed as (xR, yR), (xG, yG) and (xB, yB) on the x-y chromaticity diagram.
Specifically, this color reproducibility is expressed as the ratio of the area A to the area of a triangle (area B) defined by 3 points of red (0.67, 0.33), green (0.21, 0.71) and blue (0.14, 0.08) that are the three primary colors in the standard system prescribed by U.S. National Television System Committee (NTSC) (hereinafter referred to as NTSC ratio. The unit is % ((area A/area B)×100 (%)). The NTSC ratio is about 40 to 50% for general laptop computers, about 50 to 60% for PC monitors and about 72% for liquid crystal televisions.
The NTSC ratio increases as the color purity of each filter segment is increased.
However, as the color purity of the filter segment is increased, the light utilization efficiency of a backlight (represented by the lightness Y value) is decreased. Therefore, high electric power is required in order to increase the Y value while maintaining high color purity.
Among conventional color liquid crystal display devices, products powered mainly with a battery, such as personal digital assistants and cell phones, do not use high electric power because of an emphasis on power consumption, and thus the NTSC ratio thereof is as low as 30 to 50%.
However, as the opportunity of watching photos and TV programs on personal digital assistants and on displays of cell phones is increasing in recent years, there is increasing demand not only for lower power consumption but also for a higher NTSC ratio even in personal digital assistants and displays of cell phones.
A backlight using a cold-cathode tube as a light source is used in laptop PCs, monitors and liquid crystal televisions, while a front light or backlight using, as a light source, a white LED formed by coating the surface of a blue LED chip of lower power consumption with a layer of a phosphor which converts a blue light into a yellow light is often used in displays of products powered mainly with a battery, such as personal digital assistants and cell phones.
In the white LED, a part of the blue light emitted by blue LED passes through the phosphor layer, and the remainder is absorbed by the phosphor and converted into a yellow color. A mixed light of the two color lights of blue and yellow is recognized as white light by an observer.
That is, the light from this white LED is a mixed light of blue and yellow lights and is different in wavelength and spectrum peak from the backlight from a cold-cathode tube as a light source.
It follows that when the white LED is used, a filter segment having different spectroscopic properties from those of a filter segment used with a cold-cathode tube backlight is necessary in improving color reproducibility.
Techniques of improving color reproducibility where white LED is used are disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2004-177592 for a green filter segment and in Jpn. Pat. Appln. KOKAI Publication No. 2004-145275 for a red filter segment.
However, not only the color reproducibility of red, green and blue colors, but also the color reproducibility and lightness (Y value) of white color are important in the color liquid crystal device.
The color reproducibility and lightness (Y value) of white color cannot be improved by merely improving the color reproducibility of green and red filter segments as described in Jpn. Pat. Appln. KOKAI Publication Nos. 2004-177592 and 2004-145275 supra.
Particularly, because the red wavelength light intensity from white LED is lower than the green wavelength light intensity and blue wavelength light intensity from the LED, the red segment unlike the green segment and blue segment should not only shield a transmitted light at a wavelength higher or lower than a predetermined wavelength, but also transmit a red wavelength light with good balance.
The blue segment should shield or transmit a light in a specific wavelength range, corresponding to the characteristics of the red segment.